Such tires are subjected to repeated stresses and deformation which result in external stresses in the tire which are taken up by the various reinforcements and in particular by working plies of the crown reinforcement. These working plies usually comprise reinforcements positioned parallel to one another in one and the same ply and laid at an angle such that they cross with those of the adjacent working ply.
The function of the working plies is in particular to impart rigidity to the tire. These plies in known manner have at their ends fragile zones due to the free ends of the reinforcement elements.
It is known, in order to avoid contact between the ends of the cords of the various plies, to put in place a thickness of rubber mix which makes it possible to space apart and to decouple said free ends of the reinforcement elements of a ply from those of the adjacent ply. In the same manner, it is usual to arrange a thickness of rubber mix between the carcass ply and the end of the working ply directly positioned radially above the latter to space apart the free ends of the reinforcement elements of the working ply from those of the carcass ply. These rubber masses make it possible in particular to avoid breaks in the mixtures or cleavage for a given level of stresses.
It would appear, however, that, when using the tire and therefore following repeated deformations, incipient breaks appear in the mixture between the reinforcement elements of one and the same working ply.
Furthermore, the fact that such a tire is formed majoritarily of rubber mixes, and that it is subjected to repeated stresses and deformations, involves what may be a significant generation of heat which may result in a reduction in the life of said tire.
This phenomenon which results in an increase in temperature and risks of degradation of the tire is known and can be explained simply: the stresses exerted on the rubber mixes result in heating generated by the hysteresis of the elastomers contained in the rubber mixes. This heat generated within rubber zones cannot be evacuated sufficiently rapidly owing to the poor thermal conductivity of the polymers.
In a tire of radial type, the production of heat and the operating temperatures obtained are greatest in the regions bordering the ends of the reinforcement plies forming mainly on one hand the carcass reinforcement and on the other hand the crown reinforcement. Said temperatures, combined with high levels of stresses, have an adverse effect on the fatigue of the rubber mixes of said regions.
A reduction in said temperatures may be obtained by judicious selection of the mixtures in question: it is known that the composition of said mixtures may be selected in order to obtain hysteresis losses which are as low as possible; however, it is equally well known that these are obtained to the detriment of other properties of the same importance as a low operating temperature.
U.S. Pat. No. 4,362,200 also described the use of rubber compositions which are heat-conducting by incorporation of graphite. This choice of composition does not make it possible to avoid the generation of heat, but permits more rapid cooling of the rubber compositions. However, as previously, this choice of modification of one property of the rubber, which on one hand is very costly, on the other hand is to the detriment of other properties which are also desired.
It is furthermore known in particular from FR 1 437 569 to incorporate plies comprising reinforcement elements in the shoulders of the tires in the zones surrounding the edges of the crown reinforcement. The incorporation of such plies is intended to provide a gradual transition in terms of rigidity between the crown reinforcement, which is relatively rigid, and the carcass, which is much more flexible.